I kinda like the pope thing but wouldn't you be over-powered? Say you hold a grudge against a certain faction, wouldn't you excom that faction even if he is not 'guilty?'

I do like the idea of inviting other rulers to take the cross. I never play RTW so I don't know how you communicate with your neighbors but I guess in MTW2 agents should do more than just offering cease-fire and alliance. Kings' influence can even play a bigger role here. :)